1. Field of The Invention
The present invention relates to a semiconductor switching circuit, and more particularly to a semiconductor switching circuit which can operate at low voltages.
2. Description of the Related Art
Currently, with car telephones, portable telephones and other portable communications items, the existing radio wave frequency bands are not sufficient due to the increase in users. Herein, a number of new communications services have been started. These communications services are digital communications services which commonly use quasi microwaves (0.8 to 20 GHz).
For example, the portable telephone, which is an alternative to the conventional cordless telephone, is a telephone which uses quasi microwaves for digital communications. However, development of microwave ICs (MMIC: monolithic microwave ICs) which can be used with this frequency band has been slow. It is therefore essential that portable communications terminals are small as it is very important for them to be portable. There is therefore a significant problem with regards to the ICs used in this electronic equipment.
If it is premised that portable communications terminals are battery driven, it is inevitable that the electronic equipment used will be driven by a battery output voltage. Therefore, as battery output voltages are usually low, for example, as with a switch used in a antennae duplexer, the characteristics of the field effect transistor (FET) used will be problematic if used with a drive point which varies greatly. Namely, in order to switch this switch, the FET gate bias voltage has to be greatly varied. It is therefore necessary that when the switch is on, the signal flowing through the FET is not degraded and when the switch is off, there is no signal discharge through the FET.
Semiconductor switching circuits employed in the quasi-microwave region are disclosed in, for example, "TR switches with X band FETs connected in series and parallel" by Makoto Matsunaga et.al, in the study report MW 87-65 published by The Institute of Electronics, Information and Communication Engineers. This semiconductor switching circuit is made up of a first GaAs MESFET input into the signal circuit in series and a second GaAs MESFET input into the signal circuit in parallel. A resonant coil is then connected between the source and drain regions of these FETs. When the switching circuit is on, the first FET is turned on and the second FET is turned off. When the switch is off, the first FET is turned off and the second FET is turned on.
The same gate bias voltage .DELTA.V.sub.G is then applied to the gate electrodes of the first and second FETs. When the switching circuit is on, .DELTA.V.sub.G =0 volts and when the switching circuit is off, .DELTA.V.sub.G =-5 volts. Also, V.sub.p (the pinch off voltage) is considered to be on the order of -3 volts.
In order for the signal loss in the FET to be small, i.e. in order for the FET input loss to be small, it is necessary for the FET channel resistance to be small. The channel resistance can be shown to be I.sub.p /V.sub.p i.e. the linear region of the drain current I.sub.p against the drain voltage V.sub.D characteristic for a predetermined built in voltage V.sub.p. This FET channel resistance is small when I.sub.p /V.sub.p is large. Also, I.sub.D /V.sub.D is large when the built in voltage V.sub.b is large. It therefore follows that if the built in voltage V.sub.b is large, the FET channel resistance becomes low and the signal loss in the FET i.e. FET input loss becomes low.
In the semiconductor switching circuit in the publication disclosed above, V.sub.p is set to a large negative value. This is because if the pinch off voltage V.sub.p is set to its maximum value, .DELTA.V.sub.G can be set to its maximum value so that a low channel resistance (FET input loss) can be achieved i.e. the FET input loss is a function of the pinch off voltage V.sub.p.
In order for the semiconductor switching circuit to be turned off, it is necessary for the gate bias voltage .DELTA.V.sub.G to be lower than the pinch off voltage V.sub.p . It follows that when using the semiconductor switching circuit of the aforementioned publication in, for example, a portable telephone, a low input loss is realized. Moreover, in order for the semiconductor switching circuit to switch completely off, a power supply output voltage of about -5 volts is necessary. This is a major difficulty if the circuit is to be driven using usual battery output voltages.
Also, with Si system semiconductor elements employing a frequency of about 20 MHz it is necessary for the semiconductor switching circuit to operate at usual battery output voltages. Currently, mechanical switches made from items such as relays are used to cut the main flow but these switches are large and require a large amount of electrical power presenting the problem that the circuitry cannot be integrated.